Question

how many molecules of water are present in 18 ml of water​

Answer 1

Given :

18 ml of water or 18×$10^{-3}$ L of water

To Find :

Number of molecules of present in 18 ml of water​

Solution :

With the help of of Avogadro's hypothesis, it can be deduced that molar volume of any gaseous substance at STP is 22.4 L. So,

22.4 L of water at STP contains 6.022 × 10²³ molecules

1 L of water at STP contains $\frac{6.022 * 10^{23} }{22.4}$ molecules

so, 18×$10^{-3}$ L of water at STP will contain $\frac{6.022 * 10^{23} }{22.4}$ × 18×$10^{-3}$ molecules

                                                                 = 4.84 × 10²° molecules

∴ 4.84 × $10^{20}$ molecules is present in 18 ml of water.

Answer 2

$4.84$ ×$10^{20}$  molecules are present in 18 ml of water.

Explanation:

Given 18 ml of water  

We have to find :

Number of molecules which is present in 18 ml of water​

According to Avogadro's hypothesis-

Molar volume of any gaseous substance at STP is 22.4 L.

Thus,

⇒ $22.4 L$ of water at STP contains $6.022$ × $10^{23}$ molecules

⇒ $1 L$ of water at STP contains  molecules

so, $18ml$ of water at STP will contain × $18$ × molecules                                                                 = $4.84$×$10^{20}$ molecules

∴ $4.84$ ×$10^{20}$  molecules are present in 18 ml of water.